The GLK proteins are members of the recently categorized GARP superfamily of transcription factors (Riechmann et al., 2000) defined by G2 in maize; the Arabidopsis RESPONSE REGULATOR-B (ARR-B) proteins (Imamura et al., 1999); and the PHOSPHATE STARVATION RESPONSE1 (PSR1) protein of Chlamydomonas (Wykoff et al., 1999). In the case of G2, three of the four defining features of most transcription factors have been verified experimentally in heterologous systems. G2 is nuclearlocalized (Hall et al., 1998), is able to transactivate reporter gene expression, and can both homo-dimerize and heterodimerize with ZmGLK1 (Rossini et al., 2001). DNA-binding activity of GLK proteins has yet to be demonstrated,however, the putative DNA-binding domain is highly conserved with domains in other GARP proteins such as ARR1 and ARR2 (Riechmann et al., 2000). Notably, ARR1 and ARR2 have been shown to bind DNA (Sakai et al.,2000), thus it is likely that GLK proteins act as transcriptional regulators of chloroplast development.

The GLK proteins are members of the GARP superfamily of transcription factors, and phylogenetic analysis demonstrates that the maize, rice and Arabidopsis GLK gene pairs comprise a distinct group within the GARP superfamily. Further phylogenetic analysis suggests that the gene pairs arose through separate duplication events in the monocot and dicot lineages. As in rice, AtGLK1 and AtGLK2 are expressed in partially overlapping domains in photosynthetic tissue. GLK genes therefore regulate chloroplast development in diverse plant species.